1. Field of the Invention
This invention relates to an optical device for detecting the position of a first mark relative to a second mark by optically scanning the first and second marks on a scanning surface.
2. Description of the Prior Art
One form of such apparatus is described in U.S. Pat. No. 3,683,195, issued Aug. 8, 1972 the Karl-Heinz Johannsmeier et al, entitled "Apparatus for the automatic alignment of two superimposed objects, e.g. a semiconductor wafer and mask". According to this apparatus, mask and wafer having alignment marks on a scanning surface for semiconductor printing are superimposed and these alignment marks are scanned to detect the relative positions of alignment marks so as to detect the relative positions of the mask and wafer. The alignment mark of the mask is constituted by first and second bars parallel to each other and third and fourth bars parallel to each other and crossed with said first and second bars at a right (90) angle. The alignment mark of the wafer consists of fifth and sixth bars crossing with each other at a right (90) angle.
The scanning of these alignment marks is performed by a movable aperture disposed at the position of the enlarged image, and this movable aperture silt-shaped for increasing the level of the detecting signal.
By making the movable aperture as slits, the two slit-shape apertures crossing at a right (90) angle must be provided for detecting the crossing bars. One slit detects the position of the mark of one orientation and the other slit detects the position of the mark of the other orientation which is positioned at a right angle to said mark of one orientation.
Thus according to the prior art, taking the light amount into consideration, it is necessary to provide a plurality of the slits which complicates the structure of the optical system.